1. Field of the Invention
The present invention relates, generally, to a hydraulic circuit for an automatic transmission, and more specifically, to such a hydraulic circuit having area controlled shift actuator valves with flow force compensation.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle.
Automatic transmissions typically include a plurality of gear sets that are supported on shafts in the transmission housing. Synchronizers are used to coordinate the engagement of selected gears in the transmission. Torque is translated from the engine through the transmission at various gear ratios depending on the gear that is selected. Most automatic transmissions are controlled and actuated hydraulically. However, it has been found that controlling and regulating automatic transmissions to achieve the desired vehicle occupant comfort goals in an efficient and cost effective manner is a complicated matter. There are a large number of events to properly time and execute within the transmission for each shift to occur smoothly and efficiently.
Furthermore, since the control of automatic transmissions is carried out by hydraulically actuating the various components within the transmission, it is important to provide a stable hydraulic pressure. A pump is employed to provide pressurized hydraulic fluid for the control and actuation of the transmission. In addition, the clutches and gear assemblies are lubricated and cooled by a secondary flow of hydraulic fluid. Typically, the pump is mechanically driven by a power take-off from the engine. Thus, the hydraulic pressure delivered from the pump increases as the pump speed increases in response to an increase in engine speed. Since hydraulically actuated devices respond in a predetermined and a precise manner for the given pressure supplied to actuate them, inaccurate control of the hydraulic pressure causes inaccurate operation and control of the transmission. Up to this point, establishing and maintaining a stable hydraulic pressure in an automatic transmission has proven problematic.
The hydraulic supply circuits of conventional transmissions typically include a plurality of hydraulic valves. However, the design and operation of these valves generally fail to properly account for various flow effects of the hydraulic fluid within the hydraulic circuit and do not provide the precise and stable hydraulic pressure that is necessary to ensure accurate control over the transmission. More specifically, to provide a stable system pressure, the hydraulic valves must be responsive to changes in the flow forces that occur within the circuit due to changes in the hydraulic flow in the line pressure side and the return, or suction side of the valve.
Changes in the flow force of the hydraulic fluid can particularly affect the operation of the shift actuator valves that control actuation of the synchronizers and thus corresponding shifts among adjacent gears. The flow force is the relative force of the hydraulic fluid that acts upon the lands of the valve member as the fluid moves through the valve. The effects of these flow forces upon the shift actuator valves are manifest as the fluid flow moves through the valve body. As the hydraulic fluid moves through the valves, the inherent flow forces act against the physical surfaces of the valve member, and the applied force can physically effect the position of the valve member in the valve body causing it to move and generate instability in the valves. For example, an increase in fluid flow from the pump may act upon the valve member surfaces forcing them open further, or an increase in pump suction may cause the valve member to move in an uncontrolled manner. The movement of the valve member caused by the flow forces results in instability in the line pressure and causes further variations in the flow as the valve member tries to correct.
The conventional approaches employed with hydraulic shift actuator valves in automatic transmissions remain inefficient and susceptible to fluctuations and inaccurate control of the hydraulic pressure causing inaccurate control of the synchronizers and this can affect the smoothness of shifts in any gear in the transmission. Accordingly, there remains a need in the related art for an automatic transmission having a shift actuator valve with flow force compensation that provides stable hydraulic pressure for both steady state flow and transient flow conditions. In addition, there remains a need in the related art for an improved hydraulic circuit having a shift actuator valve wherein the area of the orifices in the shift actuator valve are opened in a controlled fashion to provide hydraulic pressure to the synchronizer to thereby better control the shift between gears while maintaining low system cost.